[0001] The present invention relates to a rear-projection system display device for reproducing
an image formed by, for example, a Cathode Ray Tube (hereinafter referred to as ÊCRTË)
or a liquid crystal panel on a transmission screen, and more particularly relates
to a projection-type display device configured so as to have a short projection distance
from projection lens to screen.
[0002] Recently there has been a general proliferation of projection-type display devices
such as projection televisions which facilitate the enjoyment of big screen viewing
by projecting an image on a screen. Devices in which a rear-projection system is used
to project image light rays from the back of the transmission screen comprising the
display unit for viewing the image on the screen are well known as such projection-type
display devices.
[0003] Figure.1 of the accompanying shows a schematic configuration of a rear-projection
system display device.
[0004] A CRT 22 as an image source for projecting the image light rays for each of the primary
colours red (R), green (G) and blue (B) (actually three CRTs 22R, 22G and 22B having
a predetermined angle for projecting each of the primary colours R, G and B, are placed
side by side) is housed in the bottom of the main body of the display device. Three
primary colour image light rays outputted from the CRTs 22 (R, G and B) and enlarged
by projection lenses 23 (R, G and B) corresponding to each of the primary colours
placed in front of the CRTs, are reflected by a reflector 24 onto the back of a transmission
screen 25.
[0005] As a result, the RGB image light rays are combined to form a colour image on the
transmission screen 25 which the viewer sees from the front.
[0006] A transparent panel 33 made of an acrylic resin such as, for example, methacrylic
resin is placed on the front of the transmission screen 25 to protect the transmission
screen 25 and enhance the luster of the displayed image.
[0007] Figure 2 is an enlarged view of a disassembled portion of the transmission screen
25 and transparent panel 33.
[0008] The transmission screen 25 comprises, for example, a Fresnel lens sheet 26 made of
an acrylic resin such as methacrylic resin and a lenticular sheet 31 for receiving
incident RGB image light rays from the direction indicated by arrow A.
[0009] The Fresnel lens sheet 26 is configured to prevent divergence of the image light
rays projected from the projection lens 23 shown in Figure 1 and to condense this
light for the viewer. To make the lens thin, the lens unit consists of many arcuate
lens elements arranged concentrically about the centre of the lens. This enables a
thin lens of large diameter to be constructed.
[0010] The lenticular sheet 31 placed behind the Fresnel lens sheet 26 is formed mainly
to enable a wider horizontal viewing angle through the horizontal dispersal and emission
of the image. Vertical black stripes 32 are formed at prescribed intervals on the
emission side of the lenticular sheet 31 to alleviate the reduction in contrast caused
by extraneous light.
[0011] Recently, in order to reduce the size of display equipment housings, the use of a
short focal length projection lens to reduce the projection distance between projection
lens and screen has been considered.
[0012] However, when the display device is configured with a short projection distance,
as is described in the following, there is a problem with reduced picture quality
due to double imaging caused by interface reflection within the Fresnel lens sheet
26.
[0013] A description of an example of the path of a beam of light falling upon a Fresnel
lens sheet 26 is given in the following.
[0014] Figure 3 is a view showing a cross section of an area of the Fresnel lens sheet 26
about 220 mm below the centre of the Fresnel lens sheet 26 and the path of a beam
of light falling upon the Fresnel lens sheet 26 in this place and in this example,
taking the distance from the projection lens 23 to the Fresnel lens sheet 26 to be
950 mm, the condensing length of the Fresnel lens sheet 26 to be 6000 mm and the refractive
index to be 1.552.
[0015] In Figure 3, the lens elements 27 comprising the Fresnel lens sheet 26 are taken
to have an angle of inclination T0a of 26.1° and a rise angle T1a of 1°.
[0016] An image light ray La1 emitted from the projection lens 23 falls on the plane of
incidence 28 of the Fresnel lens sheet 26 at an angle of incidence T2a = 13.0° and
is refracted. The image light ray Lal then falls upon a lens surface 29 at an angle
of incidence T3a = 17.7° and is emitted as an outgoing beam La2 with an angle of refraction
T4a = 2.1o. At this time, part of the image light ray La1 is reflected by the lens
surface 29 and is returned to the plane of incidence 28 as reflected light beams La3
and La4.
[0017] The reflected light beam La3 falls upon the plane of incidence 28 of the Fresnel
lens sheet 26 at an angle of incidence T5a = 43.8° and is completely reflected and
returned to a lens surface 29 so as to fall on a non-lens surface 30 at an angle of
incidence T6a = 45.3°. Then, after being completely reflected, the light falls upon
the lens surface 29 at an angle of incidence T7a = 18.7° and is refracted and emitted
downwards at an outgoing angle T8a = 55.9°. Since a case of observation from below
the bottom of the picture is extremely rare, the problem of seeing this light as a
double image is almost non-existent.
[0018] On the other hand, another reflected light beam La4, after being completely reflected
by the plane of incidence 28 of the Fresnel lens sheet 26, falls on the lens surface
29 and is completely reflected at an angle of incidence T9a = 69.9°. After this, the
light is emitted from the non-lens surface 30, but as this direction is almost parallel
with the plane of incidence 28, this light falls again on the Fresnel lens sheet 26
from the lens surface 29 and cannot be seen as a double image.
[0019] However, as previously mentioned, with a display device having a projection lens
23 with a short focal length, since the angle of incidence with respect to the Fresnel
lens sheet 26 and the angle of inclination of the lens unit has increased, double
imaging occurs readily as described in Figure 4.
[0020] Figure 4 shows an example of a case where the distance from the projection lens 23
to the Fresnel lens sheet 26 is shortened to 700 mm and the condensing length and
refractive index of the Fresnel lens sheet 26 are the same as the case described in
Figure 3.
[0021] In this description, a "b" has been added to the symbol for each part of the Fresnel
lens sheet 26.
[0022] In the case shown in Figure 4, the lens units 27b of the Fresnel lens sheet 26b are
taken to have an angle of inclination T0b of, for example, 32.7° which is larger than
T0a and with the rise angle Tlb being unchanged at, for example, 1°.
[0023] An image light ray Lb1 emitted from the projection lens 23b falls on the plane of
incidence 28b of the Fresnel lens sheet 26b at an angle of incidence T2b = 17.4° which
is larger than T2a and is refracted, then falls upon a lens surface 29b at an angle
of incidence T3b = 21.6°. Then, after refraction, the light becomes an outgoing beam
Lb2 with an outgoing angle T4b = 2.1°.
[0024] At this time, a part of the image light ray Lb1 is reflected by the lens surface
29b and is returned to the plane of incidence 28b as reflected light beams Lb3 and
Lb4.
[0025] The reflected light Lb3 returns to the plane of incidence 28b at an angle of incidence
T5b = 54.3° which is larger than T5a and is completely reflected so as to fall on
a non-lens surface 30b at an angle of incidence T6b = 34.7°.Then, without any reflection
at all, the light is emitted as an outgoing beam Lb5 with an outgoing angle T8b =
26.9°.Light emitted at this outgoing angle can then easily be viewed by eye.
[0026] In this case, the spacing "d" between the outgoing beam Lb2 which is the normal image-forming
light and outgoing beam Lb5 which comes from the reflected light Lb3, can be obtained
from

where "t" is the thickness of the Fresnel lens sheet 26b (the length of the non-lens
surface is small when compared with the plate thickness t, and the rise angle is small
at 1o, and these can therefore be ignored).
[0027] In the case of the example shown in Figure 4, if the thickness "t" is taken, for
example, to be about 2 mm, the beam spacing d becomes, for example, about 5.6 mm and
double imaging becomes noticeable in the normal image at a spacing of about, for example,
5.6 mm. Therefore, the drop in picture quality becomes particularly noticeable when
telops or movie sub-titles are displayed at the bottom of the picture.
[0028] Also, after being completely reflected by plane of incidence 28b at the back of the
Fresnel lens sheet 26b, another reflected beam Lb4 falls on a lens surface 29b at
an angle of incidence T9b = 87.0° and is completely reflected to fall on a non-lens
surface 30b at an angle of incidence T10b = 28.7°.In this case also, the incident
light is not reflected at all. Then, after being refracted, the light is emitted as
an outgoing beam Lb6 at an outgoing angle T11b = 40.8° and will be noticed as a double
image together with outgoing beam Lb5.
[0029] Figure 4 dealt with the description of a case of noticeable double imaging below
the centre of the Fresnel lens sheet 26b. However, the shape of the lens is symmetrical
in relation to the centre and similar double imaging will also therefore be noticed
from the centre in the upper area.
[0030] This double imaging is not a particular problem with respect to the areas to the
left and right because of absorption by the black stripes 32 formed on the emission
side of the lenticular sheet 31 in the stage following the Fresnel lens sheet 26b.
[0031] Figure 5 shows a table of cases of measured positions of double imaging (outgoing
beams Lb5 and Lb6) for various distances "a" (mm) from the projection lens 23 to the
Fresnel lens sheet 26, condensing length "b" (mm) on the viewing side and focal lengths
"f" (mm) of the Fresnel lens sheet 26. The thickness of the Fresnel lens sheet 26
was 2 mm and the refractive index was 1.552.
[0032] In the case of a display device with a 40-inch screen having an aspect ratio of 4:3,
the picture will be 610 mm high. Then, as described in Figure 3, in Case 1 where the
light path from the projection lens 23 to the Fresnel lens sheet 26 is set at about
950 mm, there will be no double imaging in the image displayed at a position about
250 mm away from the centre of the Fresnel lens sheet 26. Also, the image displayed
at a position about 260 mm from the centre will appear as a double image at about
255 mm but, since this is almost at the edge of the picture, it is not a particular
problem for the image.
[0033] However, as shown in Case 2 and Case 3, the appearance of double imaging occurs readily
when the light path distance "a" between the projection lens 23 and the Fresnel lens
sheet 26, and the focal length "f", are made shorter.
[0034] As shown in Case 2 for example, when the light path distance is set to a = about
800 mm, at a position separated from the centre by 220 mm or more, double imaging
tends to be displayed at a position from about 5 mm to 7 mm towards the centre from
the original image.
[0035] Moreover, as shown in Case 3, where an even shorter light path distance of a = 700
mm is set, double imaging appears in the displayed image at a position about 200 mm
away from the centre.
[0036] In other words, in Case 3, double imaging occurs at a position about 2/3 of the distance
from the centre of the Fresnel lens sheet 26 and, as mentioned above, the quality
of the image at a position away from the centre of the picture, such as at the position
of movie subtitles, is rather poor.
[0037] On this point, the occurrence of double imaging can be suppressed to the same as
that shown in Case 1 where the light path distance "a" is long if the angle of inclination
of the lens units 27 is made smaller and, as shown in Case 4, the condensing length
"b" on the viewing side is -8000 mm, i.e., if the angle of incidence of the Fresnel
lens sheet 26 lens units 27 were to be made so small that the light beam emitted from
the Fresnel lens sheet 26 is made divergent. However, the essential function of the
Fresnel lens sheet 26 would then be defeated, with luminance in the peripheral portions
of the picture reduced and the image becoming dark.
[0038] Figure 6 is a graph showing of the outgoing angles of light beams from the projection
lens 23 emerging from various points on the Fresnel lens sheet 26 in directions towards
the centre with regards to Case 3 in Figure 5. Similarly, Figure 7 applies to Case
4 and is a graph showing the outgoing angles of light beams from the projection lens
23 emerging from various points on the Fresnel lens sheet 26. In these graphs, the
vertical axes show the outgoing angles (deg) and the horizontal axes show distances
(mm) from the centre of the Fresnel lens sheet 26.
[0039] As shown in Figure 6, in Case 3 the outgoing angle (deg) towards the centre increases
in accordance with the distance from the centre and since the outgoing beam is emitted
towards the centre, the image tends to be condensed towards the viewer. However, as
shown in Figure 7, in Case 4, since the outgoing angle towards the centre (deg) becomes
more negative in accordance with the distance from the centre, the outgoing beam becomes
more diffused with distance from the centre and is directed to the outside. This causes
the previously mentioned reduction of luminance in the peripheral parts of the picture.
[0040] As described above, in a display device where the light path distance from the projection
lens 23 to the transparent screen 25 has been made short, the image light ray is concentrated
in the direction of the observer but, on the other hand, double imaging occurs at
a position 2/3 of the distance from the centre of the picture. If the angle of inclination
of the lens elements 27 of the Fresnel lens sheet 26 is made small in order to suppress
this, the luminance in the peripheral parts of the image is reduced and it therefore
becomes difficult to obtain a good image.
[0041] As the present invention sets out to resolve these kinds of problems, a projection-type
display device comprises an image projector and a screen. The image projector is for
projecting image light rays.
[0042] The screen is disposed on a side to which the image light rays are projected and
comprises a Fresnel lens sheet with one side thereof facing the image projector and
a lenticular sheet disposed on the other side of the Fresnel lens sheet for transmitting
the image light rays that are formed into a picture on the screen.
[0043] The Fresnel lens sheet has a fixed focal length in a region from the centre of the
Fresnel lens sheet to a prescribed distance and focal lengths gradually decreasing
toward the outer periphery of the Fresnel lens sheet in a region exceeding the prescribed
distance.
[0044] The prescribed distance is at least one of a distance from the centre of the Fresnel
lens sheet to a point corresponding to an upper edge of a projected image on the Fresnel
lens sheet and a distance from said centre of the Fresnel lens sheet to a point corresponding
to a lower edge of the projected image on the Fresnel lens sheet.
[0045] The Fresnel lens sheet can also be made to have focal lengths gradually decreasing
from its centre toward its outer periphery depending on the conditions of the Fresnel
lens sheet or the characteristics of the projection system.
[0046] Further, a projection apparatus comprises a housing, a screen, a reflector, an image
source and a projection lens. The screen comprises a Fresnel lens sheet and a lenticular
sheet and is disposed at a front of the housing. The reflector is disposed at the
inside rear of the housing.
[0047] The Fresnel lens sheet has a fixed focal length in a region from the centre of the
Fresnel lens sheet to a prescribed distance and focal lengths gradually decreasing
toward the outer periphery of the Fresnel lens sheet in a region exceeding the prescribed
distance.
[0048] The prescribed distance is at least one of a distance from the centre of the Fresnel
lens sheet to a point corresponding to an upper edge of a projected image on the Fresnel
lens sheet and a distance from the centre of the Fresnel lens sheet to a point corresponding
to a lower edge of the projected image on the Fresnel lens sheet.
[0049] The Fresnel lens sheet can also be made to have focal lengths gradually decreasing
from its centre toward its outer periphery depending on the conditions of the Fresnel
lens sheet or the characteristics of the projection system.
[0050] The image source can comprise a cathode ray tube or three cathode ray tubes respectively
corresponding to a red signal, a green signal and a blue signal. The image source
can also comprise a liquid crystal panel or three liquid crystal panels respectively
corresponding to a red signal, a green signal and a blue signal.
[0051] Moreover, the lenticular sheet can be made to have vertical black stripes thereon
at a prescribed interval.
[0052] Since the present invention is able to suppress double imaging appearing towards
the periphery of the picture and suppress drops in luminance at the periphery, a better
image can be provided than in the related art.
[0053] The invention will be further described by way of non-limitative example with reference
to the accompanying drawings, in which:-
Figure 1 is a view showing an example of the outline of a configuration for a projector
device of the related art;
Figure 2 is a perspective view showing a breakdown of a portion of the transmission
screen and transparent panel of Figure 1;
Figure 3 is a cross-section of part of the Fresnel lens sheet in Figure 2 about 220
mm below the centre showing the paths of light beams falling on the Fresnel lens sheet;
Figure 4 is a view showing an example of the paths of light beams falling on a Fresnel
lens sheet at about 220 mm below the centre when double imaging appears;
Figure 5 is a table of measured values of double imaging;
Figure 6 is a view of outgoing angles of light beams emitted from each point on the
Fresnel lens sheet for Case 3 shown in the table of Figure 5;
Figure 7 is a view of outgoing angles of light beams emitted from each point on the
Fresnel lens sheet for Case 4 shown in the table of Figure 5;
Figure 8 is a schematic view of a configuration of the projector device of an embodiment
of the present invention;
Figure 9 is a plan view of the Fresnel lens sheet in this embodiment;
Figure 10 is a graph showing the relationship between distance from the centre of
the Fresnel lens sheet of this embodiment and the focal length corresponding to that
distance;
Figure 11 is a view showing an example of measurements of double imaging when the
Fresnel lens sheet of this embodiment is used;
Figure 12 is a view showing an example of measurements of peripheral luminance when
the Fresnel lens sheet of this embodiment is used; and
Figure 13 is a plan view of a Fresnel lens sheet with a vertically offset centre.
[0054] The following is a description with reference to the drawings of an embodiment of
a projection-type display device of the present invention.
[0055] Figure 8 is an outline side-view of a configuration of a projector taken as the embodiment.
[0056] A projector 1 of the embodiment houses a CRT 3 as an image source for projecting
image light rays for each of the primary colours red (R), green (G) and blue (B) (actually
three CRTs 3R, 3G and 3B corresponding to each of the primary colours R, G and B having
a predetermined angle are placed side by side) in the bottom of a housing 2. Three
primary colour light images output from the CRTs 3 (R, G and B), enlarged by projection
lenses 4 (R, G and B) corresponding to each of the primary colours placed in front
of the CRTs, are reflected by a reflector 5 onto the back of a transmission screen
6.
[0057] The transmission screen 6 consists of a Fresnel lens sheet 7 and a lenticular sheet
8 configured to form an image from image light rays inputted for each of the primary
colours from the CRTs (R, G and B) via the reflector 5. In the present invention,
as described later, the focal lengths of the lens elements comprising Fresnel lens
sheet 7 are arranged to change gradually in accordance with distance from the centre.
Also, the lenticular sheet 8 has black stripes similar to the conventional example
shown in Figure 2.
[0058] The Fresnel lens sheet 7 causes the light reflected by the reflector 5 to converge,
the light becomes incident to the lenticular sheet 8, and is then dispersed horizontally
and emitted by the lenticular sheet 8. In this way, the RGB light images are combined
to form a colour image on the transmission screen 6 which the viewer sees from in
front of the screen 6.
[0059] A transparent panel 9 made of an acrylic resin such as methacrylic resin for example
is placed on the front of the transmission screen 6 to protect the transmission screen
6 and enhance the luster of displayed images.
[0060] Figure 9 is a plan view of the Fresnel lens sheet 7.
[0061] Concentric lens elements 7a, 7b, 7c, 7d... are formed at this Fresnel lens sheet
7 (although intermediate portions of these lens elements are omitted by only being
shown by broken lines, in reality, a large number of concentric rings of lens elements
are also shown at the broken line portions). In this embodiment, the focal length
of each lens element is set to a constant value from lens element 7a at the centre
to lens element 7b (200 mm from the centre for instance) and to a gradually shorter
value from lens element 7b to lens element 7d, i.e. to the outside of the Fresnel
lens sheet 7. A detailed description of setting these focal lengths is given below
in Figure 10.
[0062] Figure 10 is a graph showing the relationship between the distance from the centre
of the Fresnel lens sheet 7 and the focal length of the lens element in relation to
this distance, the horizontal axis showing the distance (mm) from the centre and the
vertical axis showing the focal length.
[0063] The distance of the Fresnel lens sheet 7 from the projection lenses 4 is 700 mm and
the refractive index of each lens element of the Fresnel lens sheet 7 is 1.55.
[0064] In Figure 10, the dashed and two-dotted line indicates an example of the focal length
(f = 627 mm) of each lens element in the case where prominent double imaging occurs
towards the edge of the image as described previously for Case 3 in Figure 5, and
the broken line indicates an example having a long focal length (f = 767 mm) for each
lens element in the case where luminance decreases due to suppression of double imaging,
as described for Case 4 of Figure 5.
[0065] In this embodiment, the focal length f for each lens element as shown by the solid
line is arranged to be almost constant from the centre to 200 or 250 mm and to become
gradually shorter towards the outer edge.
[0066] Setting a constant focal length of f = 767 mm from the centre to 200 or 250 mm enables
the occurrence of double imaging to be suppressed almost to the upper and lower edges
which are at 200 to 250 mm from the centre of the Fresnel lens sheet 7 shown in Figure
9, and to the left and right.
[0067] Also, since double imaging at the region to the left and right from 200 to 250 mm
from the centre is absorbed by the black stripes of the lenticular sheet 8 in the
stage after the Fresnel lens sheet 7, this will not appear on the screen even if the
focal length f of the lens elements is made short. Consequently, as shown in the drawings,
the focal length f of each of the lens elements can be made gradually shorter and
the decreased luminance in the peripheral parts of the image can be suppressed by
setting the focal length f so that the condensing length b becomes, for example, 6m
beyond the region about 200 to 250 mm from the centre of the Fresnel lens sheet 7.
[0068] Next, a description is given of an example of the results for a design to achieve
the focal length f (solid line) of the Fresnel lens sheet 7 in this embodiment which
was described in Figure 10.
[0069] In a general numerical expression for the configuration of a non-spherical lens:

where Z = sag, H = distance from the centre of the Fresnel lens sheet 7, K = conic
constant, C = optical axis curvature and A2 to A5 = coefficients to be optimized as
constants after performing case work.
[0070] In this embodiment, by setting the conic constant K = -1, optical axis curvature
C = -0.00234, A2 = 0, A3 = 0, A4 = -1.91 x 10
-20 and A5 = 4.43 x 10
-26, it was possible to configure the desired shape to achieve the characteristics shown
by the solid line in Figure 10.
[0071] Below is a description of an example of double imaging measurements and peripheral
luminance using the Fresnel lens sheet 7 of this embodiment.
[0072] Figure 11 is a table showing an example of measured values of double imaging using
the Fresnel lens sheet 7 of this embodiment and measured values occurring in Case
3 and Case 4 previously shown in Figure 6. In this embodiment, the condensing length
b varies between -7300 mm and 6064 mm from the centre towards the periphery and the
focal distance f of each of the lens elements is also set to vary accordingly between
774 mm and 628 mm.
[0073] Concerning the measured values of double imaging in this embodiment, as shown in
the drawings, as with Case 4, double imaging does not appear up to 250 mm from the
centre of Fresnel lens sheet 7 and is hardly ever observed in the displayed image.
[0074] Also, by making the focal length at the periphery shorter than at the centre, light
beams emitted from the periphery of the Fresnel lens sheet 7 are refracted inside
the picture even with regards to peripheral luminance, the lowering of which became
a problem in case 4 shown previously. Therefore, as shown in the table of the luminance
ratio with respect to the centre of the screen of Figure 12, it is possible to obtain
a luminance similar to that of conventional Case 3. Up to about 400 mm from the centre
the luminance in this embodiment is a little less than the luminance in Case 3 but
the difference in not discernible by the naked eye and does not affect picture quality.
[0075] In the description of the embodiment above, an example has been given where the centre
of the picture is at the centre of the Fresnel lens sheet but recently, consideration
is being given to offsetting the centre of the Fresnel lens sheet 10 from the centre
of the picture as shown in Figure 13, and offsetting the picture to the most suitable
viewing position by slanting the optical axis of the optical elements of the display
device up or down.
[0076] In the case of such a display device, focal lengths from the centre of the Fresnel
lens sheet 10 to the upper or lower edge of the picture, whichever is the more distal
according to the direction in which the centre of the picture is offset, that is to
say as far as radius Lv, are made constant so that double imaging does not occur,
with the outer focal lengths being made gradually shorter. In other words, in Figure
13, lens elements 10a and 10b to 10c are set to have a constant focal length and lens
elements from 10c to 10d... which make up the periphery are set to have gradually
shorter focal lengths.
[0077] Also, the sizes and aspect ratios of Fresnel lens sheets 7 and 10 and, depending
on the characteristics of each type of projection system (CRT, liquid crystal panel,
etc.), the focal lengths of Fresnel lens sheets 7 and 10 may be set progressively
shorter from the centres as shown by the dashed and two-dotted line in Figure 10.
[0078] In this case, by setting the conic constant K = -0.75, optical axis curvature C =
-0.00227, A2 = 0, A3 = 0, A4 = -3.78 x 10
-21, A5 = 6.93 x 10
-27, it was possible to configure the Fresnel lens sheet 10 the desired shape to achieve
the characteristics shown by the dashed and two-dotted line in Figure 10.
[0079] Also, in the description of the above embodiment, the image source is a projector
using three CRTs for each of the background colours R, G and B but the present invention
is not restricted to this. For example, the present invention can also be applied
to a liquid crystal projector device having an optical system with a single projection
lens for the enlarged projection of an image optically modulated with three liquid
crystal panels corresponding to R, G and B and then composed, or to an optical system
with a single projection lens for enlarged projection of an image optically modulated
by a single liquid crystal panel having or not having RGB colour filters.
[0080] As described above, since the projection-type display device of the present invention
enables light projected outwards from within the Fresnel lens sheet which constitutes
the transmission screen showing the projection image to be obtained at the periphery
to go in the direction of the centre by setting the focal length long from the centre
where double imaging appears almost to the upper or lower edge of the picture and
gradually shorter thereafter, even if the projection-type display device is made more
compact, reduced picture quality due to double imaging and reduced luminance at the
periphery of the Fresnel lens sheet can be suppressed and an improved image can be
obtained over the entire picture.
1. A projection-type display device comprising:
an image projector for projecting image light rays; and
a screen disposed on the side to which said image light rays is projected and comprising
a Fresnel lens sheet with one side thereof facing said image projector and a lenticular
sheet disposed on the other side of said Fresnel lens sheet for transmitting said
image light rays that are formed into a picture on said screen,
wherein said Fresnel lens sheet has a fixed focal length in a region from the
centre of said Fresnel lens sheet to a prescribed distance from the centre and focal
lengths gradually decreasing toward the outer periphery of said Fresnel lens sheet
in a region exceeding said prescribed distance.
2. A projection-type display device according to claim 1, wherein said prescribed distance
is at least one of a distance from said centre of said Fresnel lens sheet to a point
corresponding to an upper edge of a projected image on said Fresnel lens sheet and
a distance from said centre of said Fresnel lens sheet to a point corresponding to
a lower edge of said projected image on said Fresnel lens sheet.
3. A projection-type display device according to claim 1 or 2, wherein said lenticular
sheet has vertical black stripes thereon at a prescribed interval.
4. A projection-type display device comprising:
an image projector for projecting image light rays; and
a screen disposed on the side to which said image light rays is projected and comprising
a Fresnel lens sheet with one side thereof facing said image projector and a lenticular
sheet disposed on the other side of said Fresnel lens sheet for transmitting said
image light rays that are formed into a picture on said screen,
wherein said Fresnel lens sheet has focal lengths gradually decreasing from said centre
toward the outer periphery thereof.
5. A projection-type display device according to claim 4, wherein said lenticular sheet
has vertical black stripes thereon at a prescribed interval.
6. A projection apparatus comprising:
a housing;
a screen comprising a Fresnel lens sheet and a lenticular sheet and being disposed
at a front of said housing;
a reflector disposed at the inside rear of said housing;
an image source; and
a projection lens,
wherein said Fresnel lens sheet has a fixed focal length in a region from the centre
of said Fresnel lens sheet to a prescribed distance from the centre and focal lengths
gradually decreasing toward the outer periphery of said Fresnel lens sheet in a region
exceeding said prescribed distance.
7. A projection apparatus according to claim 6, wherein said prescribed distance is at
least one of a distance from said centre of said Fresnel lens sheet to a point corresponding
to an upper edge of a projected image on said Fresnel lens sheet and a distance from
said centre of said Fresnel lens sheet to a point corresponding to a lower edge of
said projected image on said Fresnel lens sheet.
8. A projection apparatus according to claim 6 or 7, wherein said image source comprises
a cathode ray tube.
9. A projection apparatus according to claim 6, 7 or 8, wherein said image source comprises
three cathode ray tubes respectively corresponding to a red signal, a green signal
and a blue signal.
10. A projection apparatus according to any one of claims 6 to 9, wherein said image source
comprises a liquid crystal panel.
11. A projection apparatus according to any one of claims 6 to 10, wherein said image
source comprises three liquid crystal panels respectively corresponding to a red signal,
a green signal and a blue signal.
12. A projection apparatus according to any one of claims 6 to 11, wherein said lenticular
sheet has vertical black stripes thereon at a prescribed interval.
13. A projection apparatus comprising:
a housing;
a screen comprising a Fresnel lens sheet and a lenticular sheet and being disposed
at a front of said housing;
a reflector disposed at the inside rear of said housing;
an image source; and
a projection lens,
wherein said Fresnel Lens sheet has focal lengths gradually decreasing from said centre
toward the outer periphery thereof.
14. A projection apparatus according to claim 13, wherein said image source comprises
a cathode ray tube.
15. A projection apparatus according to claim 13 or 14, wherein said image source comprises
three cathode ray tube respectively corresponding to a red signal, a green signal
and a blue signal.
16. A projection apparatus according to claim 13, 14 or 15, wherein said image source
comprises a liquid crystal panel.
17. A projection apparatus according to any one of claims 13 to 16, wherein said image
source comprises three liquid crystal panels respectively corresponding to a red signal,
a green signal and a blue signal.
18. A projection apparatus according to any one of claims 13 to 17, wherein said lenticular
sheet has vertical black stripes thereon at a prescribed interval.